1. Field of the Invention
The invention relates to an exhaust gas control system for an internal combustion engine, and a method for controlling the same.
2. Description of the Related Art
As a technology for reducing the amount of nitrogen oxide (NOx) discharged from an internal combustion engine, exhaust gas recirculation (EGR) for causing a portion of the exhaust gas to flow into an intake system to recirculate it back to the internal combustion engine has been suggested. There is also a technology in which a high-pressure EGR unit and a low-pressure EGR unit are provided to an internal combustion engine with a turbocharger, and EGR is performed while the EGR unit used for EGR is changed between the high-pressure EGR unit and the low-pressure EGR unit or both the high-pressure EGR unit and the low-pressure EGR unit are used in combination, based on the operating state of the internal combustion engine, as described in, for example, Japanese Patent Application Publication No. 2005-076456 (JP-A-2005-076456). The high-pressure EGR unit recirculates a portion of the exhaust gas back to the internal combustion engine through a high-pressure EGR passage that provides communication between an exhaust passage, at a portion upstream of a turbine of a turbocharger, and an intake passage, at a portion downstream of a compressor of the turbocharger. The low-pressure EGR unit recirculates a portion of the exhaust gas back to the internal combustion engine through a low-pressure EGR passage that provides communication between the exhaust passage, at a portion downstream of the turbine, and the intake passage, at a portion upstream of the compressor.
Under the low-temperature environment, for example, when the engine coolant temperature is low or when the atmospheric temperature is low, if the low-temperature air passes through an intake system, such as an intake passage, or an EGR system, such as a low-pressure EGR passage, during the fuel-supply cutoff operation, the low-temperature air is taken in the internal combustion engine when the operation mode is switched from the fuel-supply cutoff operation mode to the normal operation mode. As a result, the temperature of the intake air may excessively decrease, resulting in inconveniences such as incomplete combustion and an increase in the discharge amount of hydrocarbon (HC).
Because the low-pressure EGR passage is relatively long, it takes relatively long time until the low-temperature gas that passes through the low-pressure EGR passage during the fuel-supply cutoff operation is replaced with the exhaust gas discharged from the internal combustion engine after the operation mode is switched from the fuel-supply cutoff operation mode to the normal operation mode. Therefore, the above-described inconveniences are likely to occur in the internal combustion engine provided with a low-pressure EGR unit.
To avoid such inconveniences, for example, Japanese Patent Application Publication No. 08-232784 (JP-A-07-232784) describes the following technology. According to the technology, a bypass passage through which the intake air bypasses an intercooler is provided to an intake passage. When an internal combustion engine is under a low-temperature environment, intake air is caused to flow through the bypass passage. In this way, excessive cooling of the intake air is suppressed. However, when this technology is employed, the bypass passage and a selector valve, which changes the flow path of the intake air between the path toward the intercooler and the path toward the bypass passage, needs to be provided to an intake system. This causes inconveniences such as a reduction in ease of mounting the internal combustion engine on a vehicle and an increase in cost.